A variety of types of apparatus such as elevators, including temporary elevators and construction hoists, are conventionally equipped with overspeed brakes, the function of which is to serve as an emergency device that stops the apparatus if the linear speed exceeds a predetermined limit. If for example an elevator drive should fail, the overspeed brake prevents the cage from falling.
An elevator cage is commonly attached to a counter weight, thus raising the possibility that the cage could "fall" up as well as down. A brake used in this environment should therefore be bi-directional. Once actuated, the brake should not jam and should be easily released, preferably by reversing the direction of travel. It should also be possible for the operator to actuate the brake manually. Whether operated manually or automatically, the braking force should be applied gradually for minimum stress and reduced danger of injury. Once triggered, however, the braking force should be initiated without substantial delay.
The construction of the brake should be as simple and trouble free as possible, produce a large stopping force in comparison to its size and weight, and require minimum maintenance. It should be easily inspected without disassembly if possible and the stopping force should be readily adjustable.